Wireless communications is growing unabated. The predominant problem with respect to a continued growth is the fact that while the amount of electromagnetic spectrum available for wireless communications is limited, novel applications and an increasing number of customers necessitate an ever increasing data throughput. Thus, innovative solutions are required to meet these increasing capacity needs.
Spread spectrum communications is one of the solutions for efficiently using limited spectrum resources. In a spread spectrum transmitter, a digital bit stream at a basic data rate is spread to a transmit pulse rate, also called chip rate. This spreading operation involves applying a user unique spreading (or channelization) code to the bit stream that increases its bandwidth. Additionally, scrambling can be performed. The resulting pulse sequence (chips) is then modulated to generate an output signal. This output signal is added to other similarly processed output signals for multi-channel transmission over a communications medium. The fundamentals of spreading and scrambling are described in more detail in chapter 3.3 of Harri Holma and Antti Toskala, WCDMA for UMTS, John Wiley & Sons Ltd., second edition, 2002.
The output signals of multiple users (channels) advantageously share one transmission communications frequency, with the multiple signals appearing to be located on top of each other in both the frequency domain and the time domain. Because the applied spreading codes are user unique, however, each output signal transmitted over the shared communications frequency is similarly unique. Therefore, through the application of proper processing techniques at a receiver the individual output signals associated with individual users may be distinguished from each other.
In a spread spectrum receiver the received signals are demodulated and the appropriate digital code for the user of interest is applied to despread the desired transmitted signal and return to the basic data rate. The despreading operation thus comprises a correlation process comparing the received signal with the appropriate digital spreading code.
Modern spread spectrum receivers usually have to despread more than one spreading code simultaneously. To keep in such multi-code scenarios the complexity and power consumption of the despreading operation as small as possible, efficient techniques for jointly despreading two or more different spreading codes have to be implemented. Usually, these techniques perform quite well if the spreading codes have the same spreading factor, i.e. an identical code length.
Often, however, multi-code transmission is based on spreading codes with different spreading factors. There is thus a need for a method and a device for efficiently despreading a multi-code signal that has been generated using two or more spreading codes with different spreading factors.